:root {
    --blue-header: #7fa8d9;
    --blue-row: #dbe6f3;
    --green-btn: #a9d18e;
    --green-btn-hover: #8fc470;
    --red: #c0392b;
    --green: #2e7d32;
    --border: #b9c4d6;
    --bg: #f4f6f9;
    --text: #24313f;
}

* {
    box-sizing: border-box;
}

html, body {
    margin: 0;
    padding: 0;
    background: var(--bg);
    color: var(--text);
    font-family: -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    -webkit-tap-highlight-color: transparent;
}

.app {
    max-width: 480px;
    margin: 0 auto;
    min-height: 100vh;
    background: #ffffff;
    display: flex;
    flex-direction: column;
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.06);
}

.app-header {
    background: var(--blue-header);
    color: #12263f;
    padding: 16px;
    text-align: center;
    font-size: 1.15rem;
    font-weight: 700;
    letter-spacing: .3px;
}

.app-subheader {
    text-align: center;
    padding: 6px 12px 10px;
    font-size: .95rem;
    color: #4a5b6e;
}

.app-body {
    flex: 1;
    padding: 14px 16px;
    overflow-y: auto;
}

.app-footer {
    display: flex;
    gap: 10px;
    padding: 12px 16px 18px;
    border-top: 1px solid var(--border);
    background: #fff;
}

/* ---- Rows / fields (selection page) ---- */
.field-row {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 10px;
}

.field-label {
    flex: 0 0 34%;
    background: var(--blue-row);
    padding: 10px 8px;
    border-radius: 6px;
    font-weight: 600;
    font-size: .9rem;
    text-align: center;
}

.field-label.small {
    flex-basis: 30%;
}

select.dropdown {
    flex: 1;
    padding: 10px 8px;
    border: 1px solid var(--border);
    border-radius: 6px;
    font-size: .95rem;
    background: #fff;
    color: var(--text);
    min-width: 0;
}

select.dropdown:disabled {
    background: #f0f0f0;
    color: #999;
}

/* ---- Buttons ---- */
.btn {
    flex: 1;
    padding: 12px 10px;
    border: none;
    border-radius: 8px;
    background: var(--green-btn);
    color: #123018;
    font-weight: 700;
    font-size: .95rem;
    cursor: pointer;
    transition: background .15s ease, opacity .15s ease;
    text-align: center;
    text-decoration: none;
    display: inline-flex;
    align-items: center;
    justify-content: center;
}

.btn:hover:not(:disabled) {
    background: var(--green-btn-hover);
}

.btn:disabled,
.btn.disabled {
    opacity: .45;
    cursor: not-allowed;
    pointer-events: none;
}

.btn.faded {
    opacity: .35;
    cursor: default;
    pointer-events: none;
}

.btn.secondary {
    background: #cfd8e3;
    color: #24313f;
}

/* ---- Chapter title bar ---- */
.title-bar {
    background: var(--blue-header);
    padding: 14px 10px;
    border-radius: 8px;
    text-align: center;
    font-weight: 700;
    font-size: 1.05rem;
    margin-bottom: 10px;
}

.title-bar.small-sub {
    font-size: .85rem;
    font-weight: 500;
    margin-top: 2px;
}

/* ---- Difficulty slider ---- */
.difficulty-wrap {
    display: flex;
    align-items: center;
    gap: 8px;
    margin: 6px 0 16px;
}

.difficulty-wrap span {
    font-weight: 700;
    color: #12263f;
}

.difficulty-wrap input[type="range"] {
    flex: 1;
    accent-color: var(--red);
}

/* ---- Section list ---- */
.section-row {
    display: flex;
    align-items: center;
    background: var(--blue-row);
    border-radius: 6px;
    margin-bottom: 8px;
    overflow: hidden;
}

.section-index {
    flex: 0 0 34px;
    text-align: center;
    font-weight: 700;
    padding: 12px 4px;
    background: var(--blue-header);
    align-self: stretch;
    display: flex;
    align-items: center;
    justify-content: center;
}

.section-name {
    flex: 1;
    padding: 10px 10px;
    font-size: .92rem;
}

.section-check {
    flex: 0 0 44px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.section-check input {
    width: 20px;
    height: 20px;
}

/* ---- Question page ---- */
.question-progress {
    text-align: center;
    background: var(--blue-header);
    padding: 10px;
    border-radius: 8px;
    font-weight: 700;
    margin-bottom: 10px;
}

.question-text-box {
    background: var(--blue-row);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 16px;
    margin-bottom: 12px;
    min-height: 110px;
    font-size: 1rem;
    line-height: 1.4;
    max-height: 45vh;
    overflow-y: auto;
}

.option-row {
    display: flex;
    align-items: center;
    gap: 10px;
    background: var(--blue-row);
    border: 2px solid transparent;
    border-radius: 6px;
    padding: 10px 12px;
    margin-bottom: 8px;
    cursor: pointer;
    font-size: .95rem;
}

.option-row input[type="radio"] {
    width: 18px;
    height: 18px;
    flex-shrink: 0;
}

.option-row .option-letter {
    font-weight: 700;
    flex: 0 0 18px;
}

.option-row.selected {
    border-color: #12263f;
}

.option-row.correct-highlight {
    background: #d6f0d6;
    border-color: var(--green);
}

.option-row.wrong-highlight {
    background: #fbdada;
    border-color: var(--red);
}

.option-row.wrong-highlight .option-letter,
.option-row.wrong-highlight .option-text {
    color: var(--red);
    font-weight: 700;
}

.option-row.correct-highlight .option-letter,
.option-row.correct-highlight .option-text {
    color: var(--green);
    font-weight: 700;
}

/* ---- Results ---- */
.result-row {
    display: flex;
    align-items: center;
    gap: 8px;
    background: var(--blue-row);
    border-radius: 6px;
    margin-bottom: 8px;
    padding: 8px;
}

.result-tag {
    flex: 0 0 46px;
    text-align: center;
    font-weight: 700;
    background: #fff;
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 6px 2px;
    font-size: .85rem;
}

.result-name {
    flex: 1;
    font-size: .9rem;
}

.result-score {
    flex: 0 0 52px;
    text-align: center;
    font-weight: 700;
    font-size: .9rem;
}

.result-review-btn {
    flex: 0 0 76px;
    padding: 8px 4px;
    text-align: center;
    background: var(--green-btn);
    border-radius: 6px;
    font-weight: 700;
    font-size: .82rem;
    cursor: pointer;
}

.result-review-btn.faded {
    opacity: .35;
    cursor: default;
    pointer-events: none;
}

/* ---- Section detail table ---- */
.detail-table {
    width: 100%;
    border-collapse: collapse;
    font-size: .88rem;
}

.detail-table th {
    background: var(--blue-header);
    padding: 8px 4px;
    text-align: left;
}

.detail-table td {
    padding: 8px 4px;
    border-bottom: 1px solid var(--border);
}

.detail-table td.wrong-answer {
    color: var(--red);
    font-weight: 700;
}

.loading, .empty-state {
    text-align: center;
    color: #77879a;
    padding: 24px 8px;
}

.error-banner {
    background: #fbdada;
    color: var(--red);
    padding: 10px;
    border-radius: 6px;
    margin-bottom: 10px;
    font-size: .88rem;
    text-align: center;
}

@media (min-width: 600px) {
    .app {
        max-width: 560px;
        margin-top: 24px;
        border-radius: 12px;
        min-height: auto;
    }
}